1. Field of the Invention
The present invention relates to a method for producing an olefin homopolymer or an olefin copolymer. More particularly, the present invention is concerned with a method for producing an olefin homopolymer or an olefin copolymer from an olefin or an olefin and at least one comonomer copolymerizable therewith by continuous slurry polymerization or continuous gaseous phase polymerization in the presence of a metallocene catalyst, wherein said catalyst is subjected to hydrogen gas-treatment in which said catalyst is contacted with hydrogen gas.
The method of the present invention is advantageous in that production of an olefin homopolymer or an olefin copolymer by continuous slurry polymerization or continuous gaseous phase polymerization in the presence of a metallocene catalyst can be stably performed without suffering the occurrence of indefinite forms of polymer. That is, by the method of the present invention, a pipe for withdrawing a produced polymer from a polymerizer can be prevented from being clogged with indefinite forms of polymer, so that an olefin homopolymer or an olefin copolymer can be stably and efficiently produced by a continuous operation of a commercial scale plant.
2. Prior Art
As conventional catalysts for use in producing an olefin homopolymer or an olefin copolymer (hereinafter, both of the homopolymer and copolymer are frequently, collectively referred to simply as xe2x80x9colefin polymerxe2x80x9d), there have been known the following three types of catalysts: a titanium-containing catalyst comprising a titanium containing compound and an organoaluminum compound; a vanadium-containing catalyst comprising a vanadium-containing compound and an organoaluminum compound; and a chromium-containing catalyst.
In addition to these catalysts, as an olefin polymerization catalyst, there has recently been proposed the so-called xe2x80x9cmetallocene catalystxe2x80x9d, which comprises a metallocene compound containing a transition metal. It has been known that a metallocene catalyst is advantageous not only in that it has a high polymerization activity but also in that when the homopolymerization or copolymerization of an olefin is performed in the presence of a metallocene catalyst, there can be produced an olefin homopolymer having a narrow molecular weight distribution or an olefin copolymer having not only a narrow molecular weight distribution but also a uniform copolymerization distribution (i.e., uniform distribution with respect to the proportions of different component monomer units constituting the copolymer). It has also been known that, when the above olefin polymer having such advantageous properties has a high molecular weight, the olefin polymer exhibits excellent mechanical properties, such as high stiffness, high impact resistance and high environmental-stress-cracking resistance (ESCR). A high molecular weight olefin polymer having such excellent mechanical properties is highly promising as a molding material for producing various articles, such as bottles, pipes and films. Therefore, it has been desired to produce a high molecular weight olefin polymer by a process using a metallocene catalyst.
As representative examples of processes for polymerizing an olefin in the presence of a metallocene catalyst comprising a transition metal compound, there can be mentioned a solution polymerization process in which the polymerization is performed using a reaction solvent for a desired olefin polymer and hence the desired olefin polymer is produced in a dissolved form in the solvent, and the so-called xe2x80x9cparticulate-form type polymerization processxe2x80x9d, specifically a process in which an olefin polymer is produced in a particulate form. Examples of such particulate-form type polymerization processes include a slurry polymerization process and a gaseous phase polymerization process.
The solution polymerization process has a problem in that the viscosity of the solution of the polymer being produced is considerably increased in accordance with the increase in the molecular weight of the polymer, so that it is difficult to produce a polymer having a high molecular weight. Therefore, when it is desired to produce a high molecular weight olefin polymer by performing an olefin polymerization in the presence of a metallocene catalyst, it is necessary to employ a particulate-form type polymerization process.
On the other hand, metallocene catalysts tend to pose the following problem. A metallocene catalyst generally exhibits an especially high polymerization activity at the initial stage of the polymerization. Therefore, when a metallocene catalyst is used in a particulate-form type polymerization process, a vigorous polymerization reaction occurs in the initial stage of the polymerization, and, hence, the heat of polymerization is vigorously generated. The rate of heat generation becomes higher than the rate at which heat is removed from the reaction system. Therefore, local high temperature portions (heat spots) are formed in the particulate polymer being produced, and the temperature of such heat spots increases to a temperature which is the same as or higher than the melting temperature of the polymer. The resultant molten polymer particles fuse with each other, thereby producing indefinite forms of polymer. When such indefinite forms of polymer are formed in a continuous polymerization process, the pipe or other means for withdrawing the produced polymer from the polymerizer is clogged with the indefinite forms of polymer, so that it becomes impossible to withdraw the produced polymer from the polymerizer through the pipe, thus making it impossible to continuously perform the polymerization. When a process for producing an olefin polymer cannot be continuously performed, the efficiency of the process is low and, hence, it is difficult to practice the process on a commercial scale.
Meanwhile, it has been attempted to synthesize an excellent catalyst which is effective for solving the above-mentioned problem, and various methods have been proposed for synthesizing such excellent metallocene catalysts. However, the proposed methods for synthesizing metallocene catalysts have a problem in that the reproducibility of the polymerization behavior of the catalyst synthesized is poor. That is, even if a metallocene catalyst is synthesized by the same method, not only the polymerization activity of the catalyst obtained but also the ability of the obtained catalyst to suppress the above-mentioned problem (i.e., the occurrence of indefinite forms of polymer) changes every time the catalyst is synthesized. Therefore, it has been strongly desired to develop a particulate-form type polymerization process for producing an olefin polymer, which is advantageous in that, even when such a conventional metallocene catalyst is employed, the problem of the occurrence of indefinite forms of polymer can be reliably and surely solved, thus enabling continuous production of an olefin polymer on a commercial scale.
In this situation, the present inventor has made extensive and intensive studies with a view toward developing an improved method for producing an olefin homopolymer or an olefin copolymer, which enables stable continuous production of an olefin polymer in the presence of a metallocene catalyst. As a result, it has unexpectedly been found that, in a method for producing an olefin homopolymer or an olefin copolymer from an olefin or an olefin and at least one comonomer copolymerizable therewith by continuous slurry polymerization or continuous gaseous phase polymerization in the presence of a metallocene catalyst, for attaining the above objective, it is very effective that, wherein, said catalyst is subjected to hydrogen gas-treatment in which said catalyst is contacted with hydrogen gas. The present invention has been completed, based on the above novel finding.
It is, therefore, a primary object of the present invention to provide an improved method for producing an olefin homopolymer or an olefin copolymer, which is advantageous in that continuous production of an olefin homopolymer or an olefin copolymer by slurry polymerization or gaseous phase polymerization in the presence of a metallocene catalyst can be stably performed without the disadvantageous occurrence of indefinite forms of polymer. As a result, the polymer withdrawal pipe can be prevented from being clogged with such indefinite forms of polymer, thus enabling continuous production of an olefin homopolymer or an olefin copolymer on a commercial scale.
The foregoing and other objects, features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description taken in connection with the appended claims.